Are you expecting to have both userforms open at the same time? If so, you will need to make them modal.
If the userform reads document variables in order to populate the relevant userform on initialisation then you can simply read the document variables from one file and write them to the other file. For example
Code:
Private Sub VariableTransmission()
Dim doc1 As Document, doc2 As Document, aVar As Word.Variable
Set doc1 = ActiveDocument
Set doc2 = Documents.Add
doc1.Variables.Add "First", "Hi mum"
doc1.Variables.Add "Second", "Hello world"
For Each aVar In doc1.Variables
doc2.Variables.Add Name:=aVar.Name, Value:=aVar.Value
Next aVar
Debug.Print doc2.Variables("Second").Value
End Sub